1. Field of the invention
The present invention relates to a rigid-flexible printed circuit board (PCB) which employs a liquid crystalline polymer. More particularly, the present invention relates to a rigid-flexible PCB in which a coverlay is formed of a liquid crystalline polymer over a flexible region by an all layer processing process. Also, the present invention is concerned with a method for fabricating such a rigid-flexible PCB.
2. Description of the Prior Art
With today's ever smaller and slimmer packaging requirements, various multilayer printed circuit boards have recently been developed which can have integrated electronic devices mounted on their surfaces. Particularly, rigid-flexible PCBs are under intensive study because they have the advantage of space economy and spatial variability.
Extensively used in personal computers, PDAs and mobile phones, rigid-flexible PCBs usually comprise rigid regions which are supported by prepreg to impart mechanical strength to the PCBs, and flexible regions which connect the rigid regions to each other.
In rigid flexible circuit boards are found coverlay films which are responsible for the protection of the circuit patterns formed in the flexible regions and are usually made of polyimide.
In order to better understand the background of the invention, a description will be given of conventional rigid-flexible PCBs and their fabrication with reference to FIGS. 1 to 3, below.
FIG. 1 describes a partial coating method for forming a coverlay film on a predetermined portion of a flexible region in a rigid-flexible PCB.
First, while being masked, a copper foil layer 12 which, together with a polyimide layer 11, constitutes a polyimide copper foil laminate 10, is patterned to form a predetermined inner circuit pattern (not shown) thereon.
In order to protect the inner circuit pattern, then, a polyimide film 20 is provided as being larger in size than a flexible region within which the circuit pattern is confined.
The polyimide film 20 is placed on the flexible region of the copper foil layer 12 interleaved with an adhesive 21, followed by pseudo-bonding the polyimide film 20 to the flexible region manually with the aid of a soldering iron.
After the bonding of the polyimide film 20 to the flexible region, a prepreg 30 is provided to give mechanical strength and bonding power to the polyimide copper foil laminate 10, as shown in FIG. 1.
Afterwards, another copper foil laminate 10′ in which a polyimide layer is open is pressurized against the prepreg 30 to afford a single rigid-flexible PCB wherein the rigid regions interleaved with the prepreg 30 are connected to each other via the flexible region which is partially covered by the polyimide coverlay.
Construction of a multilayer PCB can be achieved by bonding a single rigid-flexible PCB to another interleaved with a prepreg 30′ positioned at the axis of mirror symmetry, with the aid of a press. As seen in FIG. 1, the multilayer PCB has rigid regions which are supported by prepregs 30 and 30′ to impart mechanical strength to the PCB, and flexible regions which are partially covered with polyimide film 20 and connect the rigid regions to each other.
Subsequently, etching and plating results in a via-hole for electrical interlayer connection while forming a predetermined outer circuit pattern, as shown in FIG. 3.
Next, a coverlay is formed of a polyimide film over a part of the outer circuit pattern which corresponds to the flexible region.
Afterward, the PCB is coated with a photoimageable solder resist mask ink 160 to protect the outer circuit pattern 150 as well as to prevent solder bridge formation across the outer circuit pattern 150. As a result, there is obtained a single or multi-layer rigid-flexible PCB in which coverlays made of a polyimide film are formed over the flexible region.
Such rigid-flexible PCBs, however, require a long manufacturing time and high production cost due to the coverlay processing and pseudo-bonding processes. In addition, the PCBs shows low reliability in circuit formation and stacking because of the steps caused by the partially deposited coverlay films.
To solve the problems, the overall portion of the flexible region is covered with a coverlay film, which is described with reference to FIG. 2.
First, while being masked, a copper foil layer 12 which, together with a polyimide layer 11, constitutes a polyimide copper foil laminate 10, is patterned to form a predetermined inner circuit pattern (not shown) thereon.
In order to protect the inner circuit pattern, a polyimide film 20 is bonded using an adhesive 21 onto the inner circuit pattern, covering the entire flexible region.
After the formation of the polyimide film colerlay 20 on the flexible region, a prepreg 30 is provided to give mechanical strength and bonding power to the polyimide copper foil laminate 10, as shown in FIG. 2.
Afterwards, another copper foil laminate 10′, in which a polyimide layer is open, is pressurized against the prepreg 30 to afford a single rigid-flexible PCB wherein the rigid regions supported by the prepreg 30 are connected to each other via the flexible region which is fully covered by the polyimide coverlay.
Construction of a multilayer PCB can be achieved by bonding a single rigid-flexible PCB to another interleaved with a prepreg 30′ positioned at the axis of mirror symmetry, with the aid of a press. As seen in FIG. 2, the multilayer PCB has rigid regions which are supported by prepregs 30 and 30′ to impart mechanical strength to the PCB, and flexible regions which are fully covered with polyimide film 20 and interconnect the rigid regions to each other.
Subsequently, etching and plating results in a via-hole for electrical interlayer connection while forming a predetermined outer circuit pattern, as shown in FIG. 3.
Next, a coverlay is formed of a polyimide film over a part of the outer circuit pattern which corresponds to the flexible region.
Afterward, the PCB is coated with a photoimageable solder resist mask ink 160 to protect the outer circuit pattern 150 as well as to prevent solder bridge formation across the outer circuit pattern 150. As a result, there is obtained a single or multi-layer rigid-flexible PCB in which coverlays made of a polyimide film are formed over the flexible region.
When a coverlay made of the polyimide film is deposited over the entire flexible region, the fabrication of PCB can be simplified compared to the partial coating process described in FIG. 1 because the pseudo-bonding and coverlay processing can be omitted.
However, the face of the coverlay polyimide film 20, which is not coated with the adhesive 21, has such a low surface energy due to its characteristic stiff molecular structure as to weakly bond with the prepreg 30 and 30′. Accordingly, the coverlay is easily delaminated from the prepreg 30 and 30′, which deteriorates product reliability. That is, when the polyimide film 20 for coverlay is bonded with the adhesive 21 to the flexible region, delamination between the polyimide film 20 and the adhesive 21 occurs because of the different coefficients of thermal expansion therebetween and the low thermal stability thereof.
Problems with the conventional processes also include poor dimensional stability in products and high dielectric constant in insulation layers because the polyimide film 20 has low surface energy and polar polyermic chains. Thus, it is difficult to achieve low power consumption, high frequency adoption, and slimness for conventional products.
Further, the high cost of the polyimide film impedes the price competitiveness of the conventional products.